Support is requested for an extensive histological study of the sublingual structures in prosimians and some New World monkeys. The sublingual organ was recently discovered; its nature is enigmatical. It is neither a sublingua nor a frenal lamella. On the one hand it contains the excretory ducts of the major and minor salivary glands and parts of the glands themselves; this is characteristic of the frenal lamella. On the other hand, to the latter structure it is like a sublingua, adherent to the ventral surface of the tongue. A true sublingua never contains glands or excretory ducts of glands. The specific aim of the proposed research is to solve the following problems: (1) Is the sublingual organ of Callicebus in the pattern of sublingual structures in prosimians and New World monkeys? This problem can be solved only by comparative studies comprising prosimians and those New World monkeys which are the next relatives of Callicebus. (2) What changes took place in the development of the sublingual organ in Callicebus, and what was the sequence of the single steps leading to the sublingual organ as it is found in this species? (3) What is the probable function of the sublingual organ? (4) In what species does a sublingual organ occur? It is a prerequisite to know the answer to this question in experimental tumor research, because there is a difference between a tumor of the body of the mouth, which originates from the epithelium of the glands and their derivatives, and the tumor of the tongue itself. In Callicebus it could happen that a tumor of the salivary glands would be considered a tumor of the tongue. At least the topographical location would be confusing. Therefore, it is necessary to extend this study of all those New World monkeys which are used as experimental animals.